In the course of manufacturing paper and similar products, including paperboard and the like, it is well known to incorporate quantities of inorganic materials into the fibrous web in order to improve the quality of the resulting product. In the absence of such "fillers", the resulting paper can have a relatively poor texture due to discontinuities in the fibrous web. The said fillers are also important in improving the printing qualities of the paper, i.e. by improving the surface characteristics of same. The use of appropriate such fillers, further, vastly improves the opacity and the brightness of a paper sheet of a given weight.
A number of inorganic materials have long been known to be effective for many of the aforementioned purposes. Among the best of these materials is titanium dioxide, which can be incorporated into the paper in the form of anatase or of rutile. Titanium dioxide, however, is among the most expensive materials which are so useable. Thus despite the effectiveness of such material as a filler, its use is limited and satisfactory replacements have been much sought after.
Among the materials which have found increasing acceptance as paper fillers are calcined kaolins. Materials of this type are generally prepared by calcining a crude keolin which may have been initially subjected to prior beneficiation steps in order to remove certain impurities, e.g. for the purpose of improving brightness in the ultimate product. Reference may usefully be had to Proctor U.S. Pat. No. 3,014,836, and to Fanselow et al, U.S. Pat. No. 3,586,823, which disclosures are representative of the prior art pertinent to calcined kaolins.
Those properties which render a kaolin pigment particularly valuable for use as a filler are also well known. These include a low abrasion value, and high brightness and opacifying characteristics. The low abrasion is significant in order to assure that the resultant paper product may be manufactured and processed using conventional machinery without damaging same. The brightness and opacifying characteristics are important in producing an acceptable paper sheet, one which incorporates whiteness, high opacity, good printability, and light weight.
Both the brightness characteristics of the given kaolin and the opacifying properties of same when incorporated as a filler in paper, may be quantitatively related to a property of the filler identified as the "scattering coefficient S". The said parameter, i.e. the scatterinag coefficient S of a given filler pigment, is a property well known and extensively utilized in the paper technology art, and has been the subject of numerous technical papers and the like. The early exposition of such measurements was made by Kubelka and Munk, and is reported in Z. Tech. Physik 12: 539 (1931). Further citations to the applicable measurement techniques and detailed definitions of the said scattering coefficient are set forth at numerous places in the patent and technical literature. Reference may usefully be had in this connection, e.g. to U.S. Pat. Nos. 4,026,726 and 4,028,173. In addition to the citations set forth in these patents, reference may further be had to Pulp and Paper Science Technology, Vol. 2 "Paper", Chapter 3, by H. C. Schwalbe (McGraw-Hill Book Company, N.Y.).
In order to obtain high light scattering and opacity, the major portion of filler should be in the range of one micrometer. However, good light scatter cannot be achieved solely by using a kaolin having the said size characteristics: an essential further characteristic needed is that the kaolin be structured, i.e., formed from an assemblage of platelets interconnected or bonded to provide aggregates which include a network of the platelets.
Aggregation can be of interest for additional reasons. In particular, many kaolin crude reserves are considerably finer than preferred by the paper industry; i.e., they have an unduly high proportion of particles with E.S.D.'s below 0.25 micrometers. Typical Cretaceous kaolins, e.g., include 25 to 30% by weight of particles below 0.25 micrometers; and typical Tertiary kaolins can include 50 to 60% by weight of particles below 0.25 micrometers E.S.D. (equivalent spherical diameter). The presence of such large quantities of very fine particles can have detrimental effects upon paper strength, and in other respects are undesired by the paper manufacturer. While the said extreme fines can be removed by various separation processes, such as high speed centrifuging, such removal is very expensive and entails high capital investment; this quite aside from the possible loss of high proportions of the input feed.
One method for achieving aggregation is to utilize calcining. Thus, in U.S. Pat. No. 4,381,948 to A. D. McConnell et al, a calcined kaolin pigment and a method for manufacture of same are disclosed. The said pigment consists of porous aggregates of kaolin platelets, and exhibits exceptionally high light scattering characteristics when incorporated as a filler in paper. This pigment, which substantially corresponds to the commercially available product ALPHATEX.RTM. of the present assignee, E.C.C. America, Inc. (Atlanta, Ga.), is prepared by first blunging and dispersing an appropriate crude kaolin to form an aqueous dispersion of same. The blunged and dispersed aqueous slurry is subjected to a particle size separation from which there is recovered a slurry of the clay, which includes a very fine particle size; e.g. substantially all particles can be smaller than 1 micrometer E.S.D. The slurry is dried to produce a relatively moisture-free clay, which is then thoroughly pulverized to break up agglomerates. This material is then used as a feed to a calciner; such feed is calcined under carefully controlled conditions to typical temperatures of at least 900.degree. C. The resulting product is cooled and pulverized to provide a pigment of the porous, high light scattering aggregates of kaolin platelets as described.
Calcined kaolins have also found use in paper coating applications. Reference may be had to the paper by Hollingsworth, Jones, and Bonney, "The Effect of Calcined Clays on the Printability of Coated Rotogravure and Offset Printing Papers", TAPPI Proceedings, pages 9-16, 1983 Coating Conference, discussing the advantages of incorporating small quantities of calcined kaolins into conventional kaolin-based coating formulations. Brightness and opacity of the paper both increase with increased calcined kaolin content, as may be expected from a pigment with high light scatter, and in some formulations gloss may show a slight increase with increasing calcined kaolin content.
Calcined kaolin products, including those of the aforementioned ALPHATEX.RTM. type, are seen to be manufactured by relatively complex techniques involving a multiplicity of steps, including specifically a calcining step, plus various preparatory steps and post-calcining steps. Thus, the said product is relatively expensive to produce; and requires considerable investment in complex apparatus and the like--e.g. highly regulated calciners, etc. It can indeed be noted that the conditions of preparation of these materials must be very carefully controlled in order to keep abrasion acceptably low in the calcined product. For example, the calcination operation tends per se to produce an abrasive product--in consequence of overheating--if great care is not taken to preclude such a result.
It is further to be noted that in order to produce a low abrasion calcined product, the particle size in the feed to the calciner must be carefully controlled--even a relatively small increase in coarseness of such feed can have very marked detrimental effect on Valley abrasion.
It has heretofore been known to utilize uncalcined (sometimes referred to as "hydrous")kaolin both as paper fillers and for paper coating. Because the uncalined material usually does not possess high light scattering qualities or good opacity, its usefulness, especially as a filler, is limited; and this (in addition to improving brightness) is indeed the particular advantage of calcined products of the ALPHATEX.RTM. type; i.e. by virtue of the aggregated structures of same, high light scattering properties are provided and good opacity.
With respect further to terminology, it is noted that the prior art literature, including numerous of the prior art patents relating to the field of kaolin products and processing, often uses the term "hydrous" to refer to a kaolin which has not been subjected to calcination--more specifically, which has not been subjected to temperatures above about 450.degree. C., which temperatures serve to alter the basic crystal structure of kaolin. These so-called "hydrous" clays may havebeen produced from crude kaolins, which have been subjected to beneficiation, as, for example, to froth flotation, to magnetic separation, to mechanical delamination, grinding, or similar comminution, but not to the mentiond heating as would impair the crystal structure.
In an accurate technical sense, the description of these materials as "hydrous" is, however, incorrect. More specifically, there is no molecular water actually present in the kaolinite structure. Thus, although the composition can be (and often is) arbitrarily written in the form 2H.sub.2 O.Al.sub.2 O.sub.3.2SiO.sub.2, it is now well-known that kaolinite is an aluminum hydroxide silicate of approximate composition Al.sub.2 (OH).sub.4 Si.sub.2 O.sub.5 (which equates to the hydrated formula just cited). Once the kaolin is subjected to calcination, which, for the purposes of this specification means being subjected to heating of 450.degree. C.or higher for a period which eliminates the hydroxyl groups, the crystalline structure of the kaolinite is destroyed. Therefore, such material, having been thus calcined, cannot correctly be referred to as a "kaolin". Accordingly, it should be appreciated that henceforth in this specification, when the term "kaolin" or "kaolinite" is utilized, such term necessarily implies that the original structure of the materialis intact. Thus, the term "kaolin" as used herein, can be considered to be equivalent to the technically inaccurate (but oft-occurring) prior art usage, "hydrous kaolin" or sometimes simply "hydrous clay."
From time to time, it has been proposed to provide structured kaolin agglomerates by methods unrelated to calcining, the objective being to produce a high light scattering pigment, one with good opacifying properties, without the need for calcination.
Thus, in U.S. Pat. No. 4,346,178 to Peter Economou, a structured kaolin agglomerate is disclosed wherein the clay platelets are stabilized or frozen in position by the addition thereto of a urea-formaldehyde prepolymer.
Further relevant art includes U.S. Pat. No. 4,072,537 to F. L. Kurrle. Disclosed therein is a composite silicate pigment prepared by a precipitation reaction employing an aqueous suspension of clay particles wherein spherical hydrous metal silicate particles are precipitated on the planar surfaces of clay particles having a platelet-type structure. The metal silicate pigment component is comprisedof the reaction product of a water soluble alkali metal silicate such as sodium silicate and a water soluble salt of a polyvalent metal, such as calcium chloride.
This patent is significantly different from the present invention. For example:
(a) there is no change in the particle size of the starting clay;
(b) precipitation of spherical metal silicates is stated to occur;
(c) the process is aqueous and ionically driven, i.e., induced by using calcium ions; and
(d) kaolinite is acting only as a support for spherical metal silicates.
In U.S. Pat. No. 2,296,637 to M. L. Hanahan, a sodium silicate was added to an aqueous suspension of a paper coating clay, acidified to a pH of 6 to precipitate a gel-like coating on the clay, and the suspension was filtered and dried. The process of this patent is not aggregation in the true sense, but precipitation of silica gel on a kaolinite surface, which is done prior to spray drying. It should be noted that extensive pulverization is required to disintegrate hard aggregates formed by such precipitation method. According to the patent, powder brightness increases with silicate precipitation, clearly showing the differences in the process thereof as compared with the subject aggregation method.
In U.S. Pat. Nos. 3,853,574, 3,874,140; and 3,856,545, all to Thomas H. Ferrigno, pigmentary compositions are disclosed wherein comminuted mineals such as kaolins are combined with inorganic binders such as an alkaline metal silicate, and formed into agglomerates, which when calcined produce compositions embodying numerous particles of the minerals bonded together and presenting internal voids and multiple light reflecting surfaces. The resulting pigmentary compositions are useful in applications requiring high visible light reflectance. These products, however, are, as indicated, produced by calcining.
In accordance with the foregoing, it may be regarded as an object of the present invention, to provide a structured kaolin pigment product, which possesses improved light scattering characteristics, and hence is useful as an opacifier and light scattering filler for paper and paper products, and which may similarly be used in other paper manufacturing applications, including in the coating of same.
It is also an object of the invention to provide improved coated paper and paperboard products for printing purposes such as for offset printing, gravure printing and other types of printing, using a structured kaolin pigment.
It is a further object of the present invention, to provide a pigment product of the foregoing character, which is prepared without calcination and therefore without subjecting the kaolin to high tempearatures, and which accordingly possesses low abrasiveness in accordance with the kaolin feed from which it is produced.
It is a yet further object of the invention, to provide a pigmentary product of the foregoing character, which is composed of structured aggregates of minute kaolin particles which are bonded together chemically, i.e. without the use of calcination or other high temperature techniques.
It is a still further object of the invention to provide a high light scattering pigmentary kaolin product, which may be produced at lower costs than calcined kaolins, and which can therefore provide a lower cost replacement for many applications in which calcined kaolin products or other high cost fillers are presently utilized.
It is a yet further object of the invention, to provide a process for producing a pigmentary high light scattering kaolin product, which process utilizes chemical aggregation techniques, and may be carried out without the use of a calcining step or of other high temperature processing conditions or equipment.
It is a yet further object of the present invention, to provide a process for producing structured kaolin pigments as aforementioned, which consists of a minimal number of simply conducted steps, which utilize relatively simple and inexpensive apparatus, and which introduce minimal foreign elements into the final product produced by same.
A still further object of the invention is to provide a process for producing structured kaolin pigments as aforementioned, from a feed crude containinghigh percentages of extremely minute particles, e.g., less than 0.25 micrometers, to thereby convert a crude having limited value to the paper maker to one which can be readily and economically used in paper products.
A yet further object of the invention, is to provide a process as aforementiond, which enables low abrasion structured kaolin pigments to be produced from coarser process feeds than can normally be employed in calcination to produce a product of comparable low abrasion.
It is a further object of the invention to provide bulky structured kaolin pigments which are eminently suitable for paper coating applications.
As disclosed in U.S. Ser. No. 918,632, filed Oct. 14, 1986, a fine particle size kaolin feed is reacted in particulate form with a metal chloride, such as silicon tetrachloride, to form a chemically aggregated structured kaolin pigment. Free moisture present in the particulate feed is sufficient to initiate at least a partial hydrolysis of the metal chloride or chlorides. If the moisturelevel is too high, however, it can diminish or impair the efficiency of the aggregation process. Thus in the instances where silicon and/or titanium chlorides are used, if the moisture level is too high relative to the metal chloride(s), then the hydrolysis products from the metal chlorides will predominantly precipitate as the metal oxide, i.e. as silica gel and/or titania. In the case of aluminum trichloride, if the moisture level is too high relative to the metal chloride, then the dilution of the finally resulting aluminum hydroxide (in the presence of ammonia) can be so high as to cause inefficient aggregation. In the instance of the silicon and titanium chlorides, the water to metal chloride molar ratio should be at least 0.23, and generally less than about 50, with from about 1 to 10 being a preferable molar ratio range. Where aluminum trichloride is used the lower limits of moisture are as indicated for silicon and titanium chloride; the upper limits in this case are not critical, except that when moisture level is greater than about 10% by weight of the feed, (corresponding to a water to metal chloride molar ratio of about 8.2), processing as a dry product becomes increasingly difficult, and can necessitate additional drying steps. The indicated steps are conducted under conditions such that the basic kaolinite crystalline structure (as determined by X-ray or electron diffraction) is not altered--i.e. the kaolinite is not rendered substantially amorphous, as occurs in conventional high temperature calcining as described for example in the aforementioned McConnell et al, Proctor, and Fanselow et al patents. In a further aspect, ammonia is desirably added at addition levels of from about 8 to 16 pounds per ton of the feed mixture, the ammonia being added to the previously combined kaolin and metal chloride.
In said application (also in PCT application PCT/US86/00158, see page 27, published under the number W087/00544 on Jan. 29, 1987, based on U.S. Ser. Nos. 754,475 and 802,843), it was hypothesized that the effectiveness of that invention partially may result from the hydrolysis of the silicon tetrachloride, which leads to monosilicic acid, which either reacts with the kaolin surface immediately or polymerizes and then reacts with the kaolin particles. However, the immediate reaction was thought to be more likely. Addition of ammonia helps to neutralize the acid resulting from the above, first indicated reaction, and enhances the condensation reactions of the monosilicic acid. The ammonia can also form silicon-amine type polymers, which would further enhance and bond the particles. Enhancement of the said process is effected by the addition of an enhancing agent metal ion, believed to result in the formation of metal silicates, which function as binding agents, yielding stronger aggregates, and which also helps to neutralize the acidic by-products.
Thus, in said application the aggregation of fine kaolinite is caused to occur when treated with silicon tetrachloride. These micron size, bulky aggregates are used to improve the surface properties of coated paper and the opacity of filled paper.